Color sensors are becoming more prevalent and have many potential areas of use and application. These applications include office automation, quality control and color coding in such industries as food, textile, paint, assembly and packaging, environmental lighting, consumer good, pharmaceutical, medical and research and automotive.
FIG. 5 illustrates a prior art color sensor 2 implemented on a printed circuit board 10. The prior art color sensor 2 includes a photodiode 4, a transimpedance amplifier 6, and external components (e.g., resistors) 8 that are mounted on a printed circuit board (PCB) 10. The photodiode 4 converts color light into a corresponding current that represents the incident light. The operational amplifier 6 functions as a transimpedance amplifier and generates an output voltage (V_OUT) that represents the light received by the photodiode 4. The external components 8, which can be feedback resistors, for example, may be tuned to achieve a proper gain for the amplifier 6.
FIG. 6 illustrates a prior art circuit implementation of a color sensor 2 of FIG. 5. The photodiode 4 can includes a filter and a photodiode. The filter and photodiode are referred to as a color sensor. The transimpedance amplifier includes a negative input terminal, a positive input terminal, and an output for generating the output voltage (V_OUT). The negative input terminal is coupled to one terminal of the photodiode, and the positive input terminal is coupled to a second terminal of the photodiode, which is coupled to a ground potential. The negative input terminal and the output terminal of the amplifier are coupled through a feedback resistor (R_F) and a feedback capacitor (C_F). In other words, the feedback resistor (R_F) and the feedback capacitor (C_F) are coupled in parallel between the negative input terminal and the output terminal of the transimpedance amplifier.
Since this prior art approach employs external components (e.g., the external resistors), higher photocurrents are needed to make the color sensor less sensitive to noise, which is a major consideration when designing color sensors with external components. In order to provide higher photocurrents, a larger photodiode area is required. Moreover, large external feedback resistors 8 are needed for the transimpedance amplifier 6 to converts these large currents into a corresponding output voltage. Consequently, one disadvantage of the prior art color sensor is that the sensor wastes space. For example, the large photodiode area and large resistors utilized by the prior art design is very space inefficient.
Moreover, another disadvantage of the prior art color sensor is that the sensor design employs large external resistors, whose operating characteristics are very dependent on temperature. When the operating temperature changes, the operating characteristics of the temperature dependent devices also changes, thereby complicating the circuit design and injecting problems of how to maintain a consistent output across temperature variations.
Based on the foregoing, there remains a need for a color sensing circuit that overcomes the disadvantages set forth previously.